Switch CI from GitLab to GitHub Actions #14

Job	Run time
set-image	1s
tests	1m 46s
clippy	1m 11s
checks	1m 18s
miri (bit-vec)	50m 20s
bench-rust-nightly (bit-vec)	17m 7s
miri (bytes)	2m 31s
bench-rust-nightly (bytes)	16m 15s
miri (generic-array)	2m 37s
bench-rust-nightly (generic-array)	15m 57s
miri (arbitrary)	2m 31s
bench-rust-nightly (derive)	15m 56s
bench-rust-nightly (max-encoded-len)	16m 1s
build-linux-ubuntu-amd64	1m 7s
publish-dry-run	1m 11s
	2h 25m 49s

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